How to properly add thawzone to refigeration system?

First if you are asking this question you should not be adding this. Second this stuff is a joke and will not do what it says it does. To answer your question slugging can occur if liquid enters the compressor suction valves. A compressor is designed to only pump gases. If you send liquid into the suction to fast it will not have time to evaporate causing slugging. This can seriously damage your compressor. No you can not just pour the thawzone into an open fitting.